The present invention relates to non-thermal plasma reactors and more particularly relates to scaleable inter-digitized tine non-thermal plasma reactors.
In recent years, non-thermal plasma generated in a packed bed reactor has been shown to be effective in reducing nitric oxides (NOx) produced by power plants and standby generators. These units usually have a reducing agent, such as urea, to enhance the conversion efficiency. The packed bed reactor consists essentially of a high voltage center electrode inserted into a cylinder of dielectric material, usually a form of glass or quartz. An outside or ground electrode is formed by a coating of metal in various forms, including tape, flame spray, mesh, etc. The space between the center electrode and the inside diameter of the dielectric tube is filled or packed with small diameter glass beads. When high voltage alternating current is applied to the center electrode, the surface of the beads go into corona, producing a highly reactive and selective surface for inducing the desired reaction in the gas.
Unfortunately, the packed bed design with its loose beads and glass dielectric is impractical for use in the conditions found in a mobile emitter, such as a car or truck. The vibration and wide temperature swings of the vehicle system would damage the packed bed and the necessary temperature and vibration isolation required to make it survive would not be cost effective.
Cylindrical or planar non-thermal plasma reactors are two common configurations for dielectric barrier discharge type reactors. Both of these configurations are characterized by the presence of one or more insulating layers in a current path between two metal electrodes, in addition to the discharge space.
One such reactor suitable for use with diesel engines and other engines operating with lean air fuel mixtures is disclosed in commonly assigned U.S. patent application Ser. No. 09/465,073 filed Dec. 16, 1999 entitled xe2x80x9cNon-thermal Plasma Exhaust NOx Reactor,xe2x80x9d which is hereby incorporated by reference herein in its entirety. Disclosed therein is a reactor element comprising high dielectric, nonporous, high temperature insulating means defining a group of relatively thin stacked cells forming passages and separated by insulating means. Alternate ground and charge carrying electrodes are disposed on opposite sides of the cells in the insulating means. The ground and charge carrying electrodes reside close to the cells, but are insulated therefrom by the insulating means. Such electrodes may be, for example, silver or platinum material coated onto alumina plates. Conductive ink is sandwiched between two thin nonporous alumina plates or other suitable insulating plates to prevent arcing while providing a stable electrode spacing for a uniform electric field. The electrodes are coated onto alumina in a pattern that establishes a separation between the electrodes and the connectors of alternate electrodes suitable to prevent voltage leaks.
Commonly assigned U.S. patent application Ser. No. 60/141,427 filed Jun. 29, 1999 entitled xe2x80x9cDesign and Method of Manufacturing a Plasma Reactor for Treating Auto Emissionsxe2x80x94Stacked Shapes,xe2x80x9d which is hereby incorporated by reference herein in its entirety, discloses a non-thermal plasma reactor element prepared from a planar arrangement of formed shapes of dielectric material. The shapes are used as building blocks for forming the region of the reactor wherein the plasma is generated. Individual cells are provided with a conductive print disposed on the formed shapes to form electrodes and connectors. In a preferred embodiment, the conductive print comprises a continuous grid pattern having a cutout region disposed opposite the terminal connector for reducing potential charge leakage. Multiple cells are stacked and connected together to form a multi-cell reactor element.
Commonly assigned U.S. patent application Ser. No. 09/517,681 filed Mar. 2, 2000 entitled xe2x80x9cPlasma Reactor Design for Treating Auto Emissionsxe2x80x94Durable and Low Cost,xe2x80x9d which is hereby incorporated by reference herein in its entirety, discloses a non-thermal plasma reactor element for conversion of exhaust gas constituents. The reactor comprises an element prepared from an extruded monolith of dense dielectric material having a plurality of channels separated by substantially planar dielectric barriers. Conductive material printed onto selected channels form conductive channels that are connected along bus paths to form an alternating sequence of polarity, separated by exhaust channels. Conductive channels and channels not selected for exhaust flow are plugged at end portions of the monolith with a material suitable for excluding exhaust gases and preventing electrical charge leakage between conductive channels. Exhaust channels, disposed between opposite polarity conductive channels, are left uncoated and unplugged. During operational, exhaust gas flows through exhaust channels and is treated by the high voltage alternating current plasma field. The planar shape of the dielectric barriers provides a uniform electrical response throughout the exhaust channels.
Commonly assigned U.S. patent application Ser. No. 09/517,682 filed Mar. 2, 2000 entitled xe2x80x9cMethod of Manufacture of a Plasma Reactor with Curved Shape for Treating Auto Emissions,xe2x80x9d which is hereby incorporated by reference herein in its entirety, discloses a non-thermal plasma reactor element wherein a swept shape substrate is formed and treated to create a non-thermal plasma reactor element. The substrate is formed via extrusion so that there is a series of nested, concentric dielectric barriers. Selected channels are coated with conductive material to form conductor channels capable of forming an electric field around exhaust channels. Conductive channels and channels not selected for exhaust flow are plugged at end portions of the monolith with a material suitable for excluding exhaust gases and preventing electrical charge leakage between conductive channels. Exhaust channels, disposed between opposite polarity conductive channels, are left uncoated and unplugged.
Commonly assigned U.S. Provisional Application Serial No. 60/249,231 filed November 16, entitled xe2x80x9cEdge-Connected Non-Thermal Plasma Exhaust After-Treatment Device,xe2x80x9d which is hereby incorporated by reference herein in its entirety, discloses a non-thermal plasma reactor element design where continuous edge connectors are used to provide an improved barrier to charge leakage between electrodes and between electrodes and bus paths or the housing. As a result of these improved barriers to charge leakage, electrodes can be extended up to the edge connectorxe2x80x94increasing volumetric efficiency compared to current stacked plate designs with spacers. The locations of dielectric barriers are more exact compared to earlier stacked designs using spacers, thus increasing the usable power range of the reactor with plasma present in all cells. The overall height of the subject invention is more closely controlled than for a stacked reactor design, allowing low-cost packaging techniques to be used. Also, a conductive print design is disclosed where the electrode print on the dielectric barriers extends over a large area with integral bus connection paths toward the front (or back) of the reactor. This design allows the edge connectors to be comprised entirely of a dielectric composition and without the need for vias or through slots, while eliminating the possibility of charge leakage. Bus connections are accomplished at the front (or rear) of the stack for each polarity. Subsequently, the bus paths are connected to power and ground connections. All electrical paths and connection may be covered by a dielectric encapsulent. Further a scaleable non-thermal plasma reactor element is disclosed wherein a linking edge connector is employed to efficiently join multiple elements together for increased flow treatment capability while providing key structural support for the scaled assembly.
While the above-described non-thermal plasma reactors meet some of the current needs and objectives, several problems relating to improving reactor performance, reducing manufacturing complexity, and reducing overall cost require solutions. Some of the problems to be solved include, for example, current stacked planar designs have a parting line when stacked that lies in the same plane as the metal electrode print. Due to the finite thickness of the metal electrode print as well as any camber or thickness variation that may be present in one or both of the dielectric layers, there is a gap that results between the layers. When the device is energized with high voltage, there is a tendency for charge to leak through this gapped parting line to the nearest ground path, causing thermal arcing. One known method for addressing the arcing problem, is to separate the edge of the electrode from the edge of the dielectric by a suitable distance, such as about 19 mm. This effectively reduces the potential active area of the electrode by an amount equal to the amount of separation.
Current stacked planar designs also require substantial fixturing to align pieces during assembly. Planar designs using metallized plates and discrete spacers need fixturing to hold each spacer in place relative to the metallized plates during assembly. Reactors employing formed C-shapes or box shapes eliminate the need for spacers. However, fixturing is still required to align the shapes into the stack.
Current stacked planar designs constructed from metallized plates and discrete spacers have many pieces that are relatively expensive to assemble due to excessive handling, although planar C-shapes or box shapes are less expensive to assemble.
Current stacked planar designs rely upon a stack of substrates or shapes that determine the overall height. Since each layer has a thickness variation and camber tolerance, electrode print thickness variation, and possibly burrs, there are substantial potential variations in stack height. This complicates canning the reactor substrate to withstand severe applications, such as automotive after-treatment, since variation that could exceed 10 mm is typically accommodated by custom sizing or other expensive canning methods.
Extruded monolithic substrates used as the basis for a non-thermal plasma reactor element are not prone to the parting line gap, excessive height variation, or excessive fixturing and handling problems. However, structural webs or ligaments within plasma channels, commonly necessary to impart structural integrity, can negatively affect constituent conversion efficiency.
Edge-connected non-thermal plasma reactor elements have improved reactor efficiency compared to extruded designs and increased volumetric efficiency compared to stacked designs. However, these designs rely upon many discrete dielectric barriers that must be assembled into the edge connectors, leading to increased material and assembly costs.
NOx reduction efforts are expected to increase for all mobile diesel applications, ranging from small passenger vehicle application to heavy-duty trucks, as well as for industrial stationary and temporary applications. Current stacked reactors provide poor scalability due to poor height control. While extruded cellular and edge connected reactor elements are scaleable, these reactors provide reduced performance or high material and assembly costs.
What is needed in the art is an improved non-thermal plasma reactor that can be scaled to work across a broad application mix with minimum changeover complications and expense. What is further needed in the art is a scaleable non-thermal reactor element providing improved reactor efficiency and volumetric efficiency at minimal average cost.
A scaleable inter-digitized tine non-thermal plasma reactor element is provided comprising at least one pair of inter-digitized tine end connectors connected together and defining gas passages between the tines. Alternate ground and charge carrying electrodes are provided on the tines. The prepared inter-digitized tine reactor element has a scaleable height, width, and length. Connectors are defined that enable efficient non-thermal reactor element fabrication for widely varying applications having various flow throughput and constituent reduction requirements.
The invention further provides a non-thermal plasma reactor that enables an inter-digitized tine reactor element to be constructed having several plasma zones that are selectively powered so that the effective length of the reactor can be adjusted during operation for optimal efficiency over a range of operating conditions. The invention further provides an inter-digitized tine mid connector providing efficient scaling of the reactor frontal area, wherein the frontal area has multiple electrodes provided in the exhaust flow direction to enable a variable active plasma zone for various operating conditions.
The invention further provides a structural carrier inter-digitized tine non-thermal plasma connector wherein the tines are defined in a side to side basis; that is, a high-k dielectric layer, electrode layer, structural dielectric, electrode layer, and high-k dielectric layer.
The invention further provides a structural conductor inter-digitized tine non-thermal plasma connector wherein the tines are defined in a side to side basis, that is, a high-k dielectric layer, structural conductor, and high-k dielectric layer.
The invention further provides double, single or null dielectric barrier inter-digitized tine non-thermal plasma reactors. The double dielectric barrier reactor comprises plasma cells (exhaust passages) that are bounded by a dielectric barrier in the plasma direction. The single dielectric barrier reactor comprises plasma cells that are bounded by a dielectric barrier on one side and by an electrode on the opposite side, in the plasma direction. The null dielectric barrier reactor comprises plasma cells that are bounded by electrodes on each side, in the plasma direction.
The present double dielectric inter-digitized tine reactors are prepared using structural carrier or structural conductor inter-digitized tine connectors.
The present single dielectric inter-digitized tine reactors are prepared using a structural carrier or structural conductor inter-digitized tine connector with a null dielectric barrier inter-digitized tine connector.
The present null dielectric barrier inter-digitized tine reactors are prepared using null dielectric barrier structural carrier or structural conductor inter-digitized tine connectors. In the null dielectric structural carrier inter-digitized tine connector, the tines are defined side to side as electrode layer, structural dielectric, and electrode layer. In the null dielectric structural conductor inter-digitized tine connector, the tines comprise structural conductor.